walcott.] PALEONTOLOGY — UPPER MISSISSIPPI. 221 



the plateau that extends southward from the Grand Canon. The geo- 

 graphic distribution of the formation is indicated on the atlas sheets 

 published by the Wheeler survey in 1874. 



When describing the sedimentary groups of the Plateau province 

 Maj. Powejl accepted the name Tonto group, proposed by Mr. Gilbert, 

 but did not agree with him in considering the strata to be of Silurian 

 age. From geological considerations he was inclined to consider the 

 Tonto group as forming the base of the Carboniferous series. This 

 opinion was strengthened by the fact that he found in the Grand Caiion 

 10,000 feet of sandstone, shales, and limestoues underlying the Tonto 

 series unconformably, and hence separated from them by long periods 

 of erosion, and at the base of the latter series he reported finding Silu- 

 rian fossils. 1 



The fossils mentioned by Maj. Powell were studied by Dr. C. A. 

 White, who identified the genera Lingulella and Obolella and referred 

 them to the Lower Silurian. 2 



The supposed Primordial age of the Tonto series was accepted by 

 Capt. C. A. Dutton in his account of the physical geology of the Grand 

 Caiion district. 3 



The section at the mouth of the Kanab Caiion was studied by Mr. C. 

 D. Walcott in 1880. He found that 450 feet of a mottled limestone and 

 100 feet of arenaceous and micaceous shales at the base of the section 

 were of Primordial age, as indicated by the presence of the genera Lin- 

 gulepis, Conocephalites, and Bathyurus in the upper portion, and in ad- 

 dition to these Hyolithes primordialis, Lingulepis, and Crepicephalus in 

 the lower beds. 4 Further evidence of the Cambrian age of the Tonto 

 group was published by Mr. Walcott in 1883. Fossils representing the 

 genera Cruziana, Lingulepis, Iphidea, Conocephalites, Crepicephalus, 

 and Dikelocephalus were found in the upper 700 feet of the sandstones, 

 shales, and limestones that form the upper part of the group in the 

 canon valleys entering on the left side of the Kaibab plateau. 5 



PALEONTOLOGY. 

 UPPER MISSISSIPPI AREA. 



The first reference that we find to the fossils of the Cambrian fauna 

 of the Upper Mississippi Valley is by Dr. D. D. Owen in his report of 

 1848 on a geological reconnaissance of the Chippewa land district of 



1 Report on the geology of the eastern portion of the Uinta Mountains and a region of conntry adja. 

 cent thereto. Washington, 1870, p. 56. 



2 Invertebrate paleontology of the Plateau province, together with notice of a few species from 

 localities beyond its limits in Colorado. Rept. on the geology of the eastern portion of the Uinta 

 Mountains, by J. W. Powell, Washington, 1876, p. 79. 



3 The physical geology of the Grand Canon district. U. S. Geol. Surv., 2d Ann. Rep. 1880-81. 1882, 

 p. 114. 



4 The Permian and other Paleozoic groups of the Kanab Valley, Arizona. Am. Jour.Sci., 3d ser., vol. 20, 

 1880, p. 225. 



6 Preearboniferous strata in the Grand Caiion of the Colorado, Arizona. Am. Jour. Sci., 3d ser., vol. 

 26, 1883, p. 439. 



